Wind speed measurements in upper and lower boundary layer to determine regional momentum fluxes

Radiosonde profiles in the atmospheric boundary layer (ABL) with other supp orting measurements obtained during NOPEX Second Concentrated Field Effort over a complex area in Sweden were analyzed to determine regional momentum fluxes. Two methods were explored, which consist of (i) the combined simila rity approach (CSA) for the entire ABL derived by assuming an overlap regio n between the mixed layer and surface sublayer and by joining two governing equations, and (ii) the surface sublayer profile equation derived from Mon in-Obukhov similarity theory. The CSA has an advantage in that it does not require the knowledge of the regional roughness length which itself is not easy to determine. The surface sublayer profile equation needs the roughnes s length as input. This was estimated by a model of Grant and Mason (1990) for the upwind area with the size of approximately 26 km(2), from surface i nformation such as the mean roughness height and density, with assumed drag coefficient of major obstacles, D-h/2 = 0.89, taken from previous study in a similar experimental area. A good agreement was found between the moment um fluxes derived from the two methods, and it shows the validity of the mo del used in the estimation of z(0) and the assumed value of D-h/2 = 0.89 fo r the NOPEX region. The momentum fluxes from the eddy correlation method wi th an instrument installed at 44 m on a tall tower were found to be much sm aller than those derived from the profile equation with the wind speed meas urements at the same height. The difference appears to be due to the smalle r size of the footprint area of momentum flux than that of the wind speed m easurements. (C) 1999 Elsevier Science B.V. All rights reserved.